In early 1940 at Junkers Flugzeug- und Motorenwerke A.G. wide-range preparations for series production of the new version of the Junkers Ju 88 bomber, designated A-4, which was the most produced bomber version of this aircraft began.

Development of Junkers Ju 88A bomber and reconnaissance versions Junkers Ju 88 A-2 After the start of the production of the first production version Ju 88 A-1 two subsequent versions, based on the same airframe were developed. The first of them was the Junkers Ju 88 A-2, designated, as its predecessor, as Horizontal- und Sturzbomber (horizontal and dive bomber), powered by new Junkers Jumo G-1 engines, differing from the Junkers Jumo 211 B and D engines in having strengthened block structure. The maximum take-off power of the G-1 version was 1,200 hp at 2,400 RPM. The Ju 88A-2 was the first variant adapted for use of Walter Starthilfe 109-500 A-1 take-off rocket boosters. These boosters were designed in late 1930s to facilitate the take-off of bomber aircraft carrying maximum bomb load. They were liquid-fuel devices rated at maximum thrust of 500 kG and with the burn time of around 30 seconds. The Luftwaffe personnel quickly dubbed it Kraftei (power egg). Indeed the take-off booster resembled an egg, had oval shape flattened in the forward section housing the parachute deploying automatically after the booster’s burnout and jettisoning from underwing mounts. The engine was 1.42 m long and its diameter in the widest place was 0.68 m. The rocket boosters were safe in operation, and assisted more than 3,000 documented take-offs without any serious accidents.

Junkers Ju 88A-3 Another version was the Junkers Ju 88 A-3 Schulflugzeug trainer aircraft. The prototype of this version was the Ju 88 V 16, W.Nr. 0008, D-ACAR, later BB+AD powered by Junkers Jumo 211 B engines. Production Ju 88 A-3 engines had no gun armament and bomb racks. They were equipped with double controls for the instructor and trainee.

Junkers Ju 88 A-5 The Ju 88 A-4 version, prepared for mass production was to be powered by new Junkers Jumo 211 F or J engines. The design of the Junkers Jumo 211 F engine was based was based on the Jumo 211G model, but with modified supercharger and pressure liquid cooling. Thanks to introduction of these changes the RPM increased, which allowed for attaining the maximum take-off power of 1,240 hp at 2,600 RPM. The Jumo 211 J engine was additionally fitted with an intercooler, allowing for further increase of engine’s efficiency and power. The maximum take-off power at 2,600 RPM increased to 1,420 hp.Protracting works on the start of series production of Junkers Jumo 211 F and J engines forced Reichsluftfahrtministerium to order an interim version of the Ju 88 A, designated Ju 88 A-5. Since the Ju 88 A-4 version was to be fitted with wings longer than those of A-1 through A-3 versions, the first prototype with wings of the new design was Ju 88 V17, W.Nr. 0017, with civil registration D-AEAG, later changed to military DI+MW, powered by Junkers Jumo 211 B engines. The aircraft was used for high altitude tests of new wings and modified de-icing system. The aircraft had the fuselage of Ju 88 A-1 and new wings intended for the Ju 88 A-4. Later the Ju 88 V17 prototype was fitted with barrage balloon cutter, called Kutonase.

Simultaneously at the parent plant in Dessau the last prototype of the Junkers Ju 88 built there, designated Ju 88 V18, W.Nr. 0021, civil registration D-ACAP, then BH+DA, was completed. The aircraft was used for tests with the latest Junkers Jumo 213 A engines. Production Junkers Ju 88 A-5 (Horizontal- und Sturzkampfbomber) had the standard fuselage of the A-1 version and new wings with redesigned wingtips, of span increased to 20.08 m in lieu of the wings used in the Ju 88 A-1 version, of 18.37 m span. Fabric covering of the ailerons was replaced by metal covering. The powerplant initially comprised Junkers Jumo 211 B engines, quickly replaced by Junkers Jumo 211 G, rated at 1,200 hp at 2,400 RPM. Both engines were fitted with VDM variable pitch metal propellers of 3.60 m diameter.

The fuel was located in the fuselage in two tanks, one of 1,200 liter capacity in the forward bomb bay and the other one of 680 liter capacity in the aft bomb bay and two wing tanks close to the fuselage, of total capacity of 830 l and two tanks in the outer wing sections of total capacity of 850 l. The total fuel capacity amounted to 3,580 liters. As the fuel the Einheitskraftstoff A 2 87-octane aviation gasoline was used. The coolant was contained in two armored wing tanks, each of capacity of 136 liters. Due to the increase of the coolant’s volume due to high temperature the tanks were filled to maximum 125 liters each. There was an additional 105-liter unprotected coolant tank in the port wing. The cooling system was filled with Rotring or Aero-Shell-Mittel coolant liquid.

The defensive armament initially remained unchanged and was located in three gun positions: A-Stand- in front windscreen with a single 7.92 mm MG 15 machine gun with 450 rounds, B-Stand with two single 7.92 mm MG 15 machine guns with 1,125 rounds on Linsenlafette Z 10 D flexible mounts in the rear of the cockpit canopy and C-Stand in the rear of the ventral gondola with 7.92 mm MG 15 machine gun with 600 rounds. Aircraft of later production batches had two different types of Linsenlafette Z 10 D armored mounts with square or oval observation openings. Also in the ventral gun position new types of mounts - Bola 39 C or Bola 39 D VE were installed in later production batches. A dozen or so last airplanes were fitted with Bola 81 Z mounts with twin 7.92 mm Mauser MG 81 Z machine gun. Bomber armament of Ju 88 A-5 aircraft comprised a set of bomb ejector racks and hardpoints in fuselage bomb bays and under the wings. Standard equipment of the forward bomb bay, located between the 9th and 12th bulkhead comprised two external hardpoints with four ETC 50/X ejector racks each and two internal hardpoints with five ETC 50/X ejector racks each, for eighteen 50-kg bombs (900 kg in total – Rüstsatz M 1). In the aft bomb bay located between the12th and 15th bulkhead were two external hardpoints with four ETC 50/X ejector racks each and one internal hardpoint with two ETC 50/X ejector racks for a total number of ten 50-kg bombs (500 kg - Rüstsatz M 2). Under each wing three ETC 500/IX d bomb racks could be mounted. The racks, counting from the fuselage, were marked Last I, II and III. On Last I a bomb load weighing 250 to 1,000 kg could be carried ( Rüstsatz M 3, alternatively for bombs a 900-liter drop tank could be carried on this station under each wing), on Last II bombs weighing 250 to 500 kg (Rüstsatz M 4), and on Last III a 250-kg bomb (Rüstsatz M 5) could be carried. In place of one of the ETC 500/IX d racks, an ETC 500/1000/XI rack, allowing for carrying 250-kg to 1,800 kg bomb or a 900-liter drop tank could be mounted on Last I station. The maximum load then was 1,800 kg (Rüstsatz M 8). The radio equipment comprised FuG 10 communication radio, PeilG V radio direction finder and FuBl 1 instrument landing system. For internal communication between the crewmembers the EiV intercom, interoperating with the FuG 10 radio was used. One of production Ju 88 A-5 airplanes, W.Nr. 3058, was redesignated Ju 88 V20 experimental aircraft with civil registration D-ACBN and was used for exhaust flame dampers. On 26 May 1941 the aircraft was delivered to Luftwaffe test facility at Rechlin.